This invention relates generally to permanent magnet machines, and more particularly, methods and apparatus for assembling a permanent magnet pole assembly for an electric machine.
At least some known high-torque, low-speed permanent magnet machines used with electric utility class wind generators and other applications include large diameter rotors having a diameter greater than one meter. Such rotors generally include many magnet poles. Known magnet poles are assembled from multiple magnetic blocks that are secured to the rotor. However, such designs generally do not adequately address magnet pole design issues including rotor losses, demagnetization protection, pole shaping, pole assembly, permanent magnet block capture, magnetized pole handling and/or pole mounting of a magnetized pole.
With respect to rotor losses, at least some known brushless AC synchronous permanent magnet machines may include necessary, but undesirable non-synchronous air gap flux caused by harmonics and/or sub-harmonics in the armature reaction, or by slotting. This non-synchronous flux induces eddy current loss in electrically conducting components of the rotor. Stators constructed with fractional slot windings e.g., ⅖ or 2/7 slots per pole per phase, are particularly problematic in creating non-synchronous air-gap flux, and resulting rotor losses. To facilitate preventing such rotor losses, at least some known machines utilize bonded magnets, which include magnetic powder embedded in a polymeric binder. However, the residual induction of bonded magnets is generally very low, especially for high torque machines.
Several methods of attaching magnet poles are known for example, some rotors secure the magnets to the rotor of an interior rotor machine using a fiberglass hoop that is wet wound around the magnets. On at least some rotors the magnets are held to the motor by shrunk fit metallic hoops. With this approach, the magnets are typically magnetized after they are coupled to the rotor, before the rotor is positioned relative to the stator. Other known magnet poles are magnetized prior to being inserted into an air gap defined between the pre-assembled rotor and stator. Such poles are then secured in place with separate clamping pieces. However, because the clamps are separate pieces from the magnet pole, the clamping process may be time-consuming.